Omnidirectional beacon antenna



Dec. 23, 1958 G. sTAvis ET AL 2,866,194

OMNIDIRECTIONAL BEACON ANTENNA 2 Shee'cs--Sheeil l Filed Nov. 14, 1955INVENTORS G05 5m W5 BY JAMES 5. ,5A/qa ATTORNEY Dec. z3, 1958 Filed Nov.14, 1955 G. sTAvls ET AL 2,866,194 OMNIDIRECTIONAL BEACON ANTENNA 2Sheets-Sheet 2 ATTO RN EY ie States @MNIDERECTIONAL BEACON ANTENNA GusStavis, Ossining, N. Y., and James S. Engel, Tenatly,

N. J., assignors to International Telephone and Telegraph Corporation,Nutley, N. J., a corporation of Maryland This invention relates toomnidirectional beacon antennas and, more particularly, toomnidirectional beacon 'antennas for use in producing amultiple-modulation r-adiation pattern having a fundamental modulationfrequency and one or more additional harmonics of the fundamentalmodulation frequency for use in radio navigation systems c-ommonly knownas TACAN.

Omnidirectional beacon systems such as in TACAN have a high order ofdirectional accuracy which is dependent upon the use of a directiveantenna pattern rotated at a fundamental frequency and modulated byl aharmonic of this fundamental frequency so as to produce a generallymultilobed rotating directive radiation pattern. Due to the rotation ofthe multiple-modulation antenna pattern, a receiver located remotelyfrom the transmitter receives energy which appears as anamplitudemodulated wave having a fundamental modulation oomponent and amodulation component at a harmonic frequency of the fundamental. Bothfundamental and harmonic frequency reference signals are transmittedomnidirectionally for comparison wit-h the received components of therotating pattern so that the receiver may determine its azimuth relativeto the beacons antenna system.

Previous antennas designed for use with such omnidirectional beaconshave necessitated the use of rota-ting yradio frequency (R. F.) jointsand usually have been diii'icult to enclose 4for protection from theweather. Other antenna .systems known to the prior art have disclosedthe production of the modulation frequency by the rotation of aparasitic element about a vertical stack of equally spaced centralradiators such as cones.

In many antenna systems for use with TACAN, it is desirable to utilize avertical array comprising a stack of dipoles, disc cones, biconicalradiators or a single vertical radiation element. Assuming each of theradiating elements has an equal impedance, it is desirable that each ofthe elements in a stacked vertical array be fed with currentssubstantially of predetermined phase. In the past, it has been foundthat the difference in the lengths of line necessary to couple the firstor topmost radiating element and the last or bottom element to thesource of energy has caused such vertically stacked arrays to be highlyfrequency sensitive, i. e., the radiation from the topmost element wouldbe altered in phase with respect to that from the lower elements as thefrequency has changed due to the longer length of feedline necessary tocouple the energy to the topmost antenna. It has also been founddesirable to feed such an antenna system without 'having thetransmission line lie in the radiation field of the antenna system inorder to prevent unwanted distortions of the antenna pattern.

However, it has been found that the mere excitation of a vertical stackof equally spaced central radiators such as cones produces a radiationpattern having a poor vertical-angle coverage characteristic resultingin a substantial cone of silence being present over the site of thenavigation beacon.

2,866,194 Patented. Dec. 23, 1958 One of the objects of this invention,therefore, is `to provide an improved omnidirectional beacon antennasystem especially suited for use in the TACAN naviga-tion system.

-Another object of this invention is to provide an antenna system forproducing a multilobed azimuthal directive pattern having improvedvertical-angle coverage.

Still another object of -this invention is to provide means for feedingan antenna system comprising a vertical stack of radiating elements, insuch a .manner Ias to provide proper phasing land patterncharacteristics with a minimum of radiation interference.

The above-mentioned and other features and objects of this inventionwill becomemore apparent by reference to the following description takenin conjunction with the acco-mpanying drawings, in which:

Fig. 1 is a diagrammatic illus-tration in schematic form of oneembodiment of the omnidirectional antenna system of this invention;

Fig. 2 is a partial elevational view in cross section of a portion ofthe embodiment of the TACAN antenna system shown in Fig. l; and

Fig. 3 is a perspective view partly broken away of the antenna systemand modulation or parasitic rellectors in accordance with the principlesof this invention for use in the TACAN aerial navigation system.

There is presently known in the art of aerial navigation a sys-tem ofradio navigation called TACAN in which a ground beacon transmittergenerates a constant amplitude train of pulses, including at appropriatetimes random noise pulses to maintain a constant duty cycle, andspecially coded bursts of pulses for azimuth reference se-rvice, as wellas accurately timed pulsed responses to distant measurementinterrogations. This train of tixedamplitude pulses constitutes theinpu-t to the ground beacon antenna which has for one of its mainpurposes the imposition of a certain amplitude modulation upon the pulsetrain in order to enable the airborne equipment to yield azimuthinformation. This system utilizes an antenna which is capable oflimposing a fundamental frequency amplitude modulation on the pulsetrain for a coarse azimuth indication, as well as transmitting asynchronous l5-cycle reference trigger pulse in synchronism with thefundamental amplitude modulation. In addition, transmiss-ion must beamplitude modulated at a harmonic frequency, and a harmonic frequencytrigger pulse must be radiated by the antenna in synchronism with theharmonic frequency modulation. lImportant radiation characteristics ofsuch an antenna system must be the provision for vertical directivity inthe radiation pattern to provide maximum range capabilities as well as avertical pattern uptilt to improve sight freedom. The modulatingmechanism for use in the TACAN system is complet-ely contained in theantenna of this invention and is pursely mechanical in nature,comprising basically parasitically excited wires which are rotatedaround a stationary central radiating array to obtain the desiredradiation pattern.

Referring to Figs. l and 2 of the drawings, it is seen that anomnidirecti-onal beacon antenna in accordance with the principles ofthis invention consists of a plurality of biconical dipole elements 1 7stacked in vertical alignment. Fig. 2 is a schematic illustration of aportion of the schematic diagram shown in Fig. l which is indicated bythe brackets labeled Fig. 2. This portion contains a main `or centralfeedpoint 11. The same reference characters are used in Fig. l as inFig. 2 to illustrate the identical parts. The distribution or feedsystem is contained in the central portion or mast S of the array andconsists of a number of coaxial alternately fed transmission lines, ashereinafter explained. This feed arrangement minimizes relative phaseshifts between antenna eleover the frequency range utilized. It should,of course, be understood that impedance transformers may be included atall feedpoint junctions to maintain the desired impedance.

The lower six elements 1-6 of the antenna array are equally spaced andconstitute the main array. To obtain an uptilt in the vertical pattern,the elements 1-6 are phased in such a manner that the lower two elements1 and 2 lead in current and the upper two elements 5 and 6 lag incurrent with respect to the two central elements 3 and 4. Such currentphasing causes the maximum of the major lobe of the vertical pattern tooccur somewhat above the horizon, and this effect is known to thoseskilled in the art as an uptilt of the pattern. The upper element 7 isspaced a distance different from the spacing of elements 1-6 and isutilized in order to improve the vertical-angle coverage of the antennasystem. The element 1 operates in conjunction with a counterpoise 9which is placed between elements 6 and 7. The element 7 is fed the samequantity of power as the central elements 3 and 4 and contributes to theuptilt of the vertical pattern because of the virtue of a laggingcurrent.

In order to couple energy to the radiation elements, the maintransmission line 8 comprising an inner conductor 8a and outer conductor8b proceeds to a central feedpoint 11 at which point the transmissionline divides into a distribution network. The distribution networkcontains matching transformers at all positions in order to maintain anacceptable match at all points; however, for clarity in theillustration, the transformers are not shown.

At the main feedpoint 11, the energy input is divided into two portions.The first portion comprising approximately twenty-three units of poweris coupled to a feedpoint 12 by conductor 8 functioning as an innerconductor in conjunction with conductor 13 functioning as an outerconductor. The energy of approximately twenty-three units of power atfeed-point 12 is divided into two portions of eighteen and five units ofpower each. The eighteen units of power are coupled to feedpoint 14 bymeans of conductor 13 and conductor 15 functioning as inner and outerconductors of a transmission line. The power at point 14 is evenlydivided, and nine units of energy are fed to antenna elements 3 and 4,respectively. The remaining five units of power appearing at feedpoint12 are coupled to point 16 where the power is divided s as to feedbicone 1 with one unit and bicone 2 with four units of power.

The remaining fourteen units of power are coupled from central feedpoint11 to feedpoint 17 where nine units of power are coupled to the bicone7, and the rcmaining five units are coupled to feedpoint 18 where it issplit so that four units are fed to element and one unit to element 6.This distribution of l-49 9-4l units of power can be recognized as onetending to suppress minor lobes. In order to obtain uptilt, elements 1and 2 lead and elements 5, 6 and- 7 lag in current with respect toelements 3 and 4.

Element 7 is introduced to improve the vertical-angle coverage of theantenna array. Unit 7 functions in conjunction with counterpoise 9 whichis located between elements 6 and 7. Element 7 is fed nine units ofpower and contributes to the uptilt by virtue of its lagging current. Athigh vertical angles, the counterpoise 9 tends to mask the radiationemitted from elements 1-6; and therefore, at these angles, energy isreceived predominantly from the top element.

The gain of such an antenna array is approximately 5 db over a dipole;and one desirable uptilt places the maximum lobe at a vertical angle ofapproximately 5 the reduction of siting effects. Due to the verticaldistribution of the elements 1-6 constituting the main array and thegreater spacing between the upper bicone 6 of the main array and theupper element 7 than is present between other adjacent bicones, thevertical distribution of the radiation pattern is appreciably greaterthan that due to a simple equispaced stacking of bicone antennas, thusreducing the cone of silence area enabling an aircraft ying over thebeacon to receive signal during a greater period of time.

Referring to Fig. 3, one embodiment of a TACAN antenna system is shownin perspective to comprise a base 19 which may contain the variousreference-pulse generators and mechanical movement systems used in theTACAN antenna system. The stack of biconical radiating elements arehoused in a tube of fiberglass or other insulating material. In order toproduce the fundamental signal modulation, a parasitic element 21comprising a high-resistance metallic wire is mounted upon a cylindermade of nonconductive material 22 which is then rotated about thecentral array 20. To produce the harmonic frequency modulation, aplurality of parasitic elements (not shown) are equally spaced on asecond cylinder made of a nonconductive material 23 at a greaterdistance from the array 20 than the fundamental parasitic element, andboth cylinders 22 and 23 are rotated in synchronism about the array 2G.In order to provide for desirable radiation pattern'characteristics,counterpoise plates 24 and 25 are provided. Counterpoise 25 in Fig. 3functions in a manner similar to the counterpoise 9 shown in theschematic drawing of Fig. 1.

While we have described above the principles of our invention inconnection with specific apparatus, it is to be clearly understood thatthis description is made only by way of example and not as a limitationto the scope of our invention as set forth in the objects thereof and inthe accompanying claims.

We claim:

1. An antenna array comprising a plurality of radiation elementsdisposed in a given vertical alignment, the spacing between eachradiation element of said plurality of radiation elements being a givenamount, an additional radiation element disposed in alignment withrespect to said plurality of elements and spaced therefrom at a distancedifferent than said given amount, a counterpoise disposed between saidplurality of radiation elements and said additional radiation element, asource of input energy, a transmission line coupled to said source,means to couple said energy from said transmission line to saidplurality of said radiation elements including means to divide saidenergy at a feedpoint and a plurality of branch transmission linescoupling energy from said feedpoint to each of said plurality ofradiation elements, and means to couple energy from said transmissionline to said additional radiation element.

2. An antenna array comprising a plurality of equally spaced radiationelements disposed in a vertical alignment, an additional radiationelement disposed in said vertical alignment with respect to saidplurality of radiation elements and spaced at a distance greater thansaid equal spacing, a counterpoise disposed between said plurality ofradiation elements and said additional element, a source of inputenergy, a vtransmission line coupled to said source, means to couplesaid energy from said transmission line to said plurality of elementsincluding means to divide said energy at a feedpoint and a plurality ofbranch transmission lines coupling energy from said feedpoint to each ofsaid plurality of radiation elements, and means to couple energy fromsaid transmission line to said additional radiation element. degreesabove the horizon, thus appreciably assisting in Y 3. An antenna arraycomprising a plurality of equally spaced radiation elements disposed ina vertical alignment, an additional radiation element disposed in saidvertical alignment with respect to said plurality of radiation elementsand spaced at a distance greater than said equal spacing, a counterpoisedisposed between said plurality of radiation elements and saidadditional element, a source of input energy, a transmission linecentrally disposed with respect to said vertical alignment and coupledto said source, means to couple said energy from said transmission lineto said plurality of elements including means to divide said energy at afeedpoint and a plurality of branch transmission lines coaxiallydisposed with respect to said transmission line and coupling energy fromsaid feedpoint to each of said plurality of radiation elements, andmeans to couple energy from said transmission line to said additionalradiation element.

4. An antenna array comprising a plurality of equally spaced biconicalradiation elements disposed in a given vertical alignment and anadditional biconical radiation element disposed in said verticalalignment with respect to said plurality of biconical elements andspaced at a distance greater than said equal spacing, a conductivesurface disposed between said plurality of radiation elements and saidadditional element, a source of input energy, and a plurality ofconcentric transmission lines disposed axially of said biconicalelements to couple said input energy to said radiation elements.

5. An antenna system comprising a plurality of equally spaced radiationelements disposed in a given vertical alignment and an additionalradiation element disposed in said vertical alignment and spaced at adistance greater than said equal spacing from the nearest of saidplurality of radiation elements, a conductive surface disposed betweensaid plurality of radiation elements and said additional radiationelement, a source of input energy, a plurality of concentric coaxialtransmission lines disposed axially of said biconical elements to coupleinput energy to all of said radiation elements, and at least oneparasitic element associated with said antenna array disposed forrotation about said vertical alignment.

6. An antenna system comprising a rst group of radiation elementsequally spaced and disposed in vertical alignment, at least one otherradiation element disposed above said plurality of radiators, aconductive surface disposed between said plurality and said otherradiation element, a first group of parasitic elements at a givendistance from said vertical alignment of said radiation elementsdisposed for rotation about said radiation elements, and a second groupof parasitic elements associated with said radiation elements anddisposed for rotation about said vertical alignment at a distancegreater than said given distance from said vertical alignment.

7. An antenna system according to claim 6 which further includes a iirstcylinder composed of nonconductive material surrounding said pluralityof radiation elements, said first group of parasitic elements beingcarried by said rst cylinder, a second cylinder composed ofnonconductive material surrounding said iirst cylinder, said secondgroup of parasitic elements being carried by said second cylinder, andmeans to rotate in synchronism said first and second cylinders aboutsaid radiation elements.

References Cited in the ile of this patent UNITED STATES PATENTS2,631,237 Sichak Mar. 10, 1953

